THIS INVENTION relates to water desalination, that is, to the removal of dissolved solids from sea water and brackish water.
Discussions on the world""s shortage of drinking and irrigation water are commonplace. In some parts of the world whole cities are going to have to be abandoned because of prolonged drought.
The only inexhaustible supply of water is the sea but desalination of water in significant quantities to supply major population centres or large scale irrigation projects is costly. Many desalination plants operate on the basis of reverse osmosis. In this type of plant the water to be desalinated is forced through a semi-permeable membrane so that the dissolved solids are removed by the membrane. Other plants operate on the basis of evaporation.
A major problem with both the methods described is that the water obtained is, in the case of the evaporation method, pure distilled water, and in the reverse osmosis method is of the same degree of purity as distilled water. It has virtually all the minerals that were dissolved therein removed. Water without any calcium or magnesium in it is aggressive towards metal pipes and other metal objects with which it comes into contact. Hence these minerals must be added to the recovered water. Furthermore distilled water is tasteless and, being devoid of essential minerals, cannot be used for human consumption over a prolonged period. Hence, for drinking purposes, it is necessary to add a range of minerals to convert the water from xe2x80x9cflatxe2x80x9d distilled water to acceptable drinking water. In both methods described the essential minerals which were present in the sea water are in the brine which is a by-product of the process. A significant cost in producing water from either type of plant is thus the cost of the minerals which must be reintroduced into the water and the equipment needed for this purpose.
In an evaporation plant the power needed to evaporate the sea water is also significant when costs per megalitre of recovered water are calculated.
Reverse osmosis membranes are of composite construction and one extensively used form comprises two films of a complex polymeric resin which together define a salt passage. In the passage there is an element for inducing turbulence in the flow. The element is usually a welded mesh of plastics material filaments. A number of these membranes are wound in a complex manner onto a central tube. Water which passes through said films enters spaces between adjacent membranes and flows to the central tube. The tube has apertures in the wall thereof to permit the recovered water to enter the tube. The brine, that is, the residue of the sea water and the bulk of the dissolved solids flows out of the multitude of salt passages to waste or to a salt recovery plant.
It is accepted by those working in this art that on each side of each salt passage, and immediately adjacent each film, there is a concentration polarization layer. These layers, which are of multi-molecular thickness, contain a higher concentration of dissolved solids than the bulk flow in the part of the salt passage mid-way between the films. The turbulence inducing element is intended to reduce the thickness of the concentration polarization layer and hence enhance the ability of the membrane to allow water to permeate through it. Typically a state of the art reverse osmosis membrane will achieve a 99.3% dissolved solids rejection rate. The dissolved solids that pass through the membrane largely consist of common salt as its molecules are smaller than the molecules of most other minerals. A percentage of 0.7% represents 400-500 parts per million of dissolved solids in the recovered water, depending on the initial salinity of the sea water, and is below the threshold at which the dissolved solids impart taste to the water.
Fouling of reverse osmosis membranes is a major problem and measures which increase the cost of water production have to be taken to inhibit fouling and to remove it when it does occur. Fouling can result from mineral deposition in the membrane or from organic growth. By way of example, before the sea water reaches the membrane it is treated with an inhibitor such as sodium hexametaphosphate (known commonly as xe2x80x9cshrimpxe2x80x9d). This limits calcium and magnesium precipitation on the membrane in the form of calcium and magnesium carbonates but adds another factor to production costs.
Membrane manufacturers recommend a relatively low flux rate (rate of water flow through a membrane in litres per hour per square metre of membrane) to avoid rapid fouling. Back-washing of a membrane, that is, causing water to flow in the reverse direction through the salt passages, is a standard procedure for removing fouling. If a membrane is heavily fouled it must be removed from the recovery plant and subjected to a variety of treatments for the purpose of removing the fouling. In extreme cases the fouling cannot be removed and the membrane has to be discarded.
As a result of all these factors water produced from a reverse osmosis plant is more costly than water obtained by purifying water from a storage dam or river. Hence, despite the world""s shortage of water, only a small percentage of the world""s water is produced using reverse osmosis plants to desalinate sea water.
The main objects of the present invention are to improve the efficiency of the reverse osmosis process, significantly to reduce the cost of water produced by the reverse osmosis process, to inhibit fouling of reverse osmosis membranes and to produce water with desirable minerals therein without the necessity for dosing.
According to one aspect of the present invention there is provided a reverse osmosis desalination plant which comprises a filter element consisting of reverse osmosis membranes defining salt passages, a pump for pumping water to be desalinated to said filter element, and an obstruction in the water flow path between said pump and said filter element for introducing turbulence into the flowing water and causing a pressure drop across the obstruction whereby the water downstream of the obstruction as it enters said salt passages of the filter element is at a lower pressure than the water upstream of the obstruction and its flow is more turbulent than it was upstream of the obstruction.
The obstruction is preferably in the form of a plate with a plurality of holes in it whereby the flowing water is obstructed and divided up into a number of conical, diverging turbulent water streams each of which is at a lower pressure than the pressure of the water upstream of the plate. The holes in the plate can be of different sizes or can all be of the same size as one another. In a preferred form the plate is in the form of a circular disc and the holes are in a spiral array about the centre of the disc. In another form the holes are in a circular array and in yet another form the holes lie along lines radiating out from the disc centre.
If desired a series of flow restricting valves can be provided for varying the flow areas of the holes in the plate which create the individual water streams.
According to a further aspect of the present invention there is provided a method of desalinating water which comprises pumping water to be desalinated to a filter element consisting of reverse osmosis membranes defining salt passages, causing a pressure drop in the water flowing to the filter element and simultaneously introducing turbulence into the water flow, and feeding the turbulent water at the lower pressure into the salt passages of the filter element.
In the preferred embodiment the water is divided into a plurality of turbulent conically shaped, diverging water streams by said obstruction which drops the pressure and introduces the turbulence, each turbulent stream impinging on the filter element.
It has been found that inlet pressures in the range 50 to 65 Bar and a pressure drop of between 1.5 and 2.0 Bar provide the best results.
The plant and method according to the present invention recover water which has acceptable levels of dissolved solids, that is, minerals in it. No dosing of the recovered water is required as it has therein sufficient dissolved solids to give it an acceptable taste. Because magnesium and calcium are present in the recovered water it is not aggressive towards metal pipes and fittings and no dosing with these minerals is required.
It is believed that by introducing water which is flowing in a turbulent manner into the salt passages of the membranes, the concentration polarization layer is reduced in thickness. This enables the flux rate to be increased without unduly increasing fouling. A further effect is to allow through the membrane minerals in addition to common salt whilst not increasing the quantity of common salt in the recovered water to an unacceptable level. Experimental work has shown that by varying the pressure drop and the turbulence, for example by varying the hole sizes in the plate when this forms the obstruction, different dissolved solids can be caused to pass through the membranes in controllable quantities. Hence by trial and experiment i.e. by varying the pressure drop and turbulence, water having dissolved solids in predetermined quantities can be recovered.
A further advantage is that experimental work has shown that fouling of the membrane is significantly reduced when turbulent water is fed to it.
The brine which emerges from a conventional reverse osmosis plant is heavier than sea water and hence sinks if fed back into the sea. However, the brine emerging from a desalination plant in accordance with the present invention, when fed back into the sea, initially rises in the form of a plume instead of sinking. The brine has been found to be aerated, and the aerating agent has been found to be oxygen. Furthermore, there are oxygen bubbles in the recovered water.
Tests show that there is more oxygen in the recovered water and in the brine than there should be based on the amount of oxygen dissolved in sea water. The oxygen bubbles are small because, even downstream of the obstruction, there is substantial pressure, for example, 45 to 50 Bar. The small bubbles in the turbulent water are believed to play a part in reducing the thickness of the concentration polarization layers. The bubbles also seem to play a part in preventing fouling of the membrane.